Biosynthesis and Secretion of Amyloid P Component in Rat Liver

Abstract

Amyloid P component was isolated from rat liver and serum; its properties, biosynthesis, and glycosylation in the liver were investigated. The molecular weights of intracellular and serum amyloid P component were estimated to be 28, 000 and 30, 500, respectively. The two forms were immunologically identical, and kinetic study revealed a clear precursor-product relationship between them. The total mRNA was prepared from rat liver with or without turpentine treatment, and the RNA content of amyloid P component was estimated by the incorporation of [³⁵S] methionine into the in vitro translation products. The turpentine treatment induced a marked increase in the level of translatable mRNA of the amyloid P component (_??_46 fold), while the serum level of the protein elevated only moderately (_??_1.7 fold). Most of the intracellular amyloid P component was sensitive to endo H. Various subcellular fractions were prepared from rat livers previously labeled in vivo with [³⁵S] methionine. The protein prepared from the rough and smooth microsomes and heavy Golgi fraction were all sensitive to endo H, whereas that from the light Golgi fraction was a mixture of forms sensitive to and resistant to endo H. This result suggests that the processing of the mannosyl oligosaccharide chains and the subsequent addition of terminal sugars to convert the liver amyloid P component (28, 000 daltons) to serum counterpart (30, 500 daltons) were performed in the trans-Golgi region just before secretion of the amyloid P component.